If all of your integers are small enough to fit into that size of numeric column, then just use alter column, here is an example:

create table intTable (col1 int)

insert into intTable
values (1)

insert into intTable
values (2)

go

alter table intTable
alter column col1 numeric(3, 2)

However, if you try it with values that are too large, it will give you an arithmetic overflow error. In that case, you either need to increase the size of the numeric type you are converting it to or remove the large values first.